A non-contact charging method has been developed in which a battery-powered device incorporating a power receiving coil is set down on a charging platform incorporating a power transmission coil, power is transmitted from the power transmission coil to the power receiving coil, and the battery in the battery-powered device is charged (see Patent Document 1).
In this non-contact charging method, the battery-powered device is set down on the charging platform so that the receiving coil in the battery-powered device becomes electromagnetically coupled with the power transmission coil in the charging platform. When this occurs, power is transmitted from the power transmission coil to the power receiving coil, and the battery is charged using power induced in the power receiving coil. This charging method does not require connecting the battery-powered device to the charging platform via a connector, and a built-in battery can be charged conveniently using this non-contact method.
When a foreign object such as a metal paper clip has been set down on the charging platform when a battery-powered device is being charged, the induced current flows through the foreign object and it may become hot due to the joule heat. Because induced current flowing through a foreign object is a waste of power, the battery set down on the charging platform is not charged efficiently. In order to eliminate this problem, a plurality of temperature sensors has been installed side-by-side inside the upper surface of the charging platform of Patent Document 1 in both the vertical and horizontal directions. These temperature sensors detect heat from a foreign object set down on the charging platform. When a metal foreign object has been set down on the charging platform, and alternating current power is supplied to the power transmission coil, induced current flows to the foreign object, and the heat generated by the foreign object is detected by the temperature sensor closest to the object.
For example, see Japanese Laid Open Patent Publication No. 2008-17562.
A plurality of temperature sensors has to be installed in the upper surface of the charging surface in order to determine whether a foreign object has been set down. The required number of temperature sensors increases component costs. Also, because a given temperature sensor can detect heat generated by a foreign object only when the foreign object has been set down near the temperature sensor, a foreign object can only be detected when temperature signals are received from all of the temperature sensors. Therefore, the detection circuit required to receive temperature signals from a large number of temperature sensors is complicated. As a result, a foreign object cannot be detected using a simple circuit configuration.
Because a large number of temperature sensors have to be installed in the charging platform on which the battery-powered device is set down, the temperature sensors and the components used to install the temperature sensors are located between the power transmission coil and the power receiving coil. This increases the interval between the power transmission coil and the power receiving coil, and decreases the power transmission efficiency. Because the power transmission coil and the power receiving coil are electromagnetically coupled to transmit power, the interval between the coils has to be reduced in order to perform more efficient power transmission. However, a configuration in which temperature sensors are installed between the power transmission coil and the power receiving coil does not allow the power receiving coil to be brought closer to the power transmission coil. This reduces power transmission efficiency.
The present invention was developed to solve this problem. The most important purpose of the present invention is to provide a non-contact charging method in which the setting down of a foreign object on the charging platform can be detected stably and reliably using a simple circuit configuration, and in which power can be transmitted more efficiently from the power transmission coil to the power receiving coil.
In the non-contact charging method of the present invention, a battery-powered device 50 is set down on a charging platform 10, a power receiving coil 51 in the battery-powered device 50 is electromagnetically coupled to a power transmission coil 11 in the charging platform 10, power is transmitted from the power transmission coil 11 to the power receiving coil 51 by electromagnetic induction, and a battery 52 in the battery-powered device 50 is charged. Also, in the non-contact charging method, the charging current of the battery 52 is detected on the battery-powered device 50 side, the transmission efficiency is detected from the detected charging current, the detected transmission efficiency is compared to an efficiency threshold value, it is determined that a foreign object has been set down on the charging platform 10 when the detected charging efficiency is less than the efficiency threshold value and it is determined that a foreign object has not been set down on the charging platform 10 when the detected charging efficiency is greater than the efficiency threshold value, and foreign object detection is suspended during a time period including a fluctuation in the charging current.
The battery is charged by the power induced in the power receiving coil. Thus, the charging current of the battery is the output of the power receiving coil, that is, the power outputted from rectifier circuit converting the alternating current power from the receiving coil into direct current power. Herein, the “charging current of the battery” is used interchangeably to include the current outputted from the rectifier circuit, and the current outputted from the rectifier circuit can be detected as the charging current of the battery.
In this non-contact charging method, the setting down of a foreign object on the charging platform can be reliably detected using a simple circuit configuration. This is because the battery-powered device can detect the charging current of the battery, detect the transmission efficiency, and compare the detected transmission efficiency to an efficiency threshold value to detect a foreign object. The method can detect when some of the power from the power transmission coil is consumed by a foreign object, thus reducing the power induced in the power receiving coil and reducing the transmission efficiency. Because a large number of temperature sensors does not have to be installed in the charging platform, as in the prior art, the structure and electronic circuitry of the charging platform can be greatly simplified. Because the current detection circuit inside the battery-powered device used to determine when the battery has been completely charged can also be used to detect the charging current, foreign objects can be detected without requiring a special circuit to detect the charging circuit. In this method, a foreign object is detected by detecting the transmission efficiency based on the charging current of the battery. As a result, a foreign object can be detected as soon as it is set down on the charging platform.
In this non-contact charging method, erroneous detection of a foreign object based on fluctuations in the load connected to the battery can also be prevented. Because a foreign object is detected by detecting the transmission efficiency based on the charging current of the battery and then comparing the detected transmission efficiency to an efficiency threshold value, fluctuations in the charging current, and thus transmission efficiency, can cause a foreign object to be erroneously detected. However, in this non-contact charging method, foreign object detection is suspended in a time period including a fluctuation in the charging current. This prevents erroneous detection of a foreign object under these conditions. In other words, foreign objects can be detected stably and reliably.
In the non-contact charging method of the present invention, when a fluctuation occurs in the charging current of the battery 52 and the charging current temporarily rises above or falls below the current change detection threshold value, the period of time including the rise or fall in the charging current above or below the current change detection threshold value is determined to be a time period including a fluctuation in the charging current when the period of time is shorter than a set time period, and foreign object detection is suspended during this time period.
In this non-contact charging method, erroneous detection of a foreign object based on fluctuations in the load connected to the battery can also be prevented while also detecting foreign objects using the transmission efficiency detected based on the charging current of the battery. In a method detecting a foreign object based on the transmission efficiency, a foreign object may be erroneously detected when there is a fluctuation in the load of the battery and the charging currency of the battery fluctuates. For example, when the load of a charged battery falls or the charging of the battery is temporarily stopped on the battery-powered device side, the charging current to the battery, and thus the transmission efficiency, may fall. In a method detecting a foreign object using the transmission efficiency detected from the charging current, the setting down of a foreign object may be erroneously detected even when a foreign object has not been set down. However, in this non-contact charging method, foreign object detection is suspended when the period in which the charging current rises above or falls below the current change detection threshold value, that is, the time in which the charge current fluctuates, is shorter than the set period. As a result, a foreign object is not erroneously detected due to fluctuations in the load of the battery. Because the drop in transmission efficiency is continuous when a foreign object has been set down, the setting down of a foreign object is determined when the transmission efficiency is continuously below the efficiency threshold value. Therefore, the setting down of a foreign object is not erroneously detected when the transmission efficiency temporarily falls below the efficiency threshold value due to a load fluctuation. When the charging current has become constant again, foreign object detection from the transmission efficiency is resumed. Thus, the setting down of foreign objects can be reliably detected.
In the non-contact charging method of the present invention, when a charging current greater than the current change detection threshold value falls below the current change detection threshold value or a charging current less than the current change detection threshold value rises above the current change detection threshold value, a predetermined time period can be set as the time period including a fluctuation in the charging current in the time period once the charging current passes the current change detection threshold value, and foreign object detection can be suspended during this time period.
In the non-contact charging method of the present invention, the charging current for the battery 52 is detected on the battery-powered device 50 side, the detected charging current and the efficiency threshold value are transmitted to the charging platform 10, and the charging platform 10 performs foreign object detection from the charging current and the efficiency threshold value. Because, in this non-contact charging method, foreign object detection is performed using the charging current for the battery and efficiency threshold value provided by the battery-powered device, the supply of power to the power transmission coil can be controlled when a foreign object has been detected, and heating of the foreign object can be prevented.
In the non-contact charging method of the present invention, the power supplied to the power transmission coil 11 is controlled when a foreign object is detected on the charging platform 10. In this non-contact charging method, the supply of power to the power transmission coil can be reduced or the supply of power to the power transmission coil can be suspended when a foreign object has been detected, and hazards such as the heating of a foreign object can be reliably prevented.
In the non-contact charging method of the present invention, the efficiency threshold value used to compare the detected transmission efficiency can be set to a lowest threshold value, and foreign object detection is effectively stopped during a time period including a fluctuation in the charging current. This non-contact charging method can substantially suspend detection of foreign objects by reducing the efficiency threshold value.
Also, in the non-contact charging method of the present invention, the lowest threshold value can be level 0. This non-contact charging method can substantially suspend detection of foreign objects by changing the efficiency threshold value.
In the non-contact charging method of the present invention, the supply of power to the power transmission coil 11 can be stopped and charging of the battery 52 stopped, or the power introduced to the power transmission coil 11 can be reduced below a power setting, and the amount of charging current for the battery 52 reduced when a foreign object is detected. This method prevents heating of a foreign object set down on the charging platform, or allows the battery in a battery-powered device to be charged while reducing the heating of the foreign object.